B7 Family
Positive and negative co-stimulatory signals play critical roles in the modulation of lymphocyte activity, and the molecules that mediate these signals have proven to be effective targets for immunomodulatory agents. For example, upon interaction with B7-1 or B7-2 on the surface of antigen-presenting cells (APC), CD28, the prototypic T cell co-stimulatory molecule, emits signals that promote T cell proliferation and differentiation in response to T cell receptor (TcR) engagement, while the CD28 homologue cytotoxic T lymphocyte antigen-4 (CTLA-4) mediates inhibition of T cell proliferation and effector functions. (See Chambers et al., Ann. Rev. Immunol., 19:565-594, 2001; Egen et al., Nature Immunol., 3:611-618, 2002.)
Several molecules with homology to the B7 family have been discovered (Abbas et al., Nat. Med., 5:1345-6, 1999; Coyle et al., Nat. Immunol., 2: 203-9, 2001; Carreno et al., Annu. Rev. Immunol., 20: 29-53, 2002; Liang et al., Curr. Opin. Immunol., 14: 384-90, 2002), and their role in lymphocyte activation is just beginning to be elucidated. These new co-stimulatory counter-receptors include B7h2, PD-L1, PD-L2, B7-H3 and B7-H4.
The expression of known B7 family members is largely restricted to antigen-presenting cells. Studies have revealed that B7 family members are counter-receptors on lymphoid cells that interact with cognate receptors on lymphocytes to provide positive or negative co-stimulatory signals that play critical roles in the regulation of cell-mediated immune responses.
NK Cells and NKp30
Natural killer (NK) cells are a subset of lymphocytes active in the immune system and represent an average of about 15% of mononuclear cells in human peripheral blood. NK cells were initially described functionally in 1971 by the observation that lethally irradiated mice were capable of rejecting allogeneic or parental strain bone marrow cell (BMC) allografts. (See Cudowicz and Bennett, J. Exp. Med. 134:83-102, 1971; Cudowicz and Bennett, J. Exp. Med. 135:1513-1528, 1971.) Cudowicz and Bennett observed that irradiated F1 hybrid H-2-heterozygous mice (A×B) were capable of rejecting parental H-2-homozygous BMC (A or B). This observation conflicted with the classic laws of transplantation in which transplantation antigens were thought to inherit co-dominantly and offspring were obligately tolerant toward parental major histocompatability complex (MHC) determinants (See Cudowicz and Bennett, J. Exp. Med. 134:83-102, 1971.) The cells responsible for this phenomenon were found to be radioresistant and identical to lymphoid cells, which were characterized later in 1975 by their ability to mediate spontaneous killing of tumors in vitro in an MHC-unrestricted manner (See Herberman and Ortaldo, Science, 214:24-30, 1981; Ortaldo and Herberman, Annu. Rev. Immunol. 2:359-394, 1984; Trinchieri, Adv. Immunol. 47:187-376, 1989; Murphy et al., J. Natl. Cancer Inst. 85:1475-1482, 1993.) Additional evidence that NK cells alone could mediate the specificity of marrow graft rejection emerged in 1987 when it was observed that mice with severe combined immune deficiency (SCID), which cannot develop T and B cells, have normal NK cell function. (See Murphy et al., J. Exp. Med. 165:1212-1217, 1987.)
NK cells are currently understood to represent an important arm of innate immunity and to play a primary role in immune surveillance against tumors and virally infected cells. Unless activated, however, NK cells are ineffective in performing their normal function, even when present in otherwise sufficient numbers. Indeed, decreased NK cell activity is associated with cancer and infectious diseases (see Yamazaki et al., Oncology Reports 9:359-363, 2002; Rosenberg et al., Cancer Research 51:5074-5079 (suppl.), 1991; Britteenden et al., Cancer 77:1226-1243, 1996; U.S. Pat. Nos. 5,082,833 and 4,883,662). Conversely, as noted above, NK cell activity mediates acute rejection of BMC allografts. Therefore, levels of NK cell activity appear to play an important role in immune-related disorders.
NK cell activity is typically regulated by the interaction between MHC class I molecules and inhibitory and activating receptors. (See, e.g., Barao and Murphy, BB&MT 9:727-741, 2003.) The “missing self” hypothesis is originally based on the observation that tumor cells that lack MHC class I molecules are susceptible to killing by NK cells. (See Ljunggren and Kane, Immunol. Today 11:237-244, 1990; Ohlen et al., J. Immunol. 145:52-58, 1990.) Investigators additionally observed that human NK cells lyse class-I-deficient Epstein-Barr-virus-transformed B-lymphoblastoid cell lines. (Storkus et al., Proc. Natl. Acad. Sci. USA 86:2361-2364, 1989.) Also, it was found that transfection of class I genes into class I-deficient target cells caused these cells to be partially or completely resistant to NK cell-mediated lysis. (See Storkus et al., supra; Shimizu and DeMars, Eur. J. Immunol., 19:447-451, 1989.) MHC class I, however, is not always necessary for protection from NK-cell-mediated cytotoxicity, and recognition by MHC class I does not always prevent cytolysis by NK cells. (Barao and Murphy, supra.) During recent years, various MHC-class-I-specific inhibitory and activating receptors as well as non-MHC-class-I-specific activating receptors have been identified. These receptors are relevant with respect to therapeutic approaches such as, e.g., allogeneic BMT and cancer therapy. (See id.)
Non-MHC-class-I-specific activating receptors, which are capable of mediating NK cell cytotoxicity against MHC-class-I-deficient or negative targets, are represented in part by a heterogeneous family of NK cell-specific immunoglobulin-like molecules that are known as natural cytotoxicity receptors (NCRs). (See, e.g., Moretta et al., Annu. Rev. Immunol. 19:197-223, 2001; Diefenbach and Raulet, Immunol. Rev., 181:170-184, 2001.) In the absence of MHC class I expression (such as, for example, on tumor cells or virus-infected cells), ligation of these activating receptors on NK cells triggers target-cell killing. One such activating receptor is NKp30, which is selectively and constitutively expressed on mature natural killer (NK) cells and signals through, inter alia, coupling with CD3ζ. (See Barao and Murphy, supra.) The target-cell ligand to which NKp30 binds has not been previously identified.
This system of innate recognition by NK cells represents a potentially powerful tool for clinical application in allogeneic bone marrow transplantation (BMT), cancer therapy, or treatment of other NK-cell-associated disorders. (See, e.g., Barao and Murphy, supra.) For example, stimulating or inhibiting activation of NKp30 would be useful for modulating NK cell activity and treating diseases or disorders associated with NK cell activity. In particular, enhancement of NK cell activity by triggering NKp30 would be useful for treatment of diseases or disorders characterized by insufficient NK cell activity, such as cancer and infectious disease, while inhibition of NK cell activity by blocking NKp30 would be useful for treating NK-cell-mediated disorders, such as, for example, BMC allograft rejection.
The newest member of the B7 family, B7H6, was recently discovered and characterized as a counter-receptor for NKp30, a receptor selectively expressed on mature natural killer (NK) cells and which is involved in human natural cytotoxicity as an activatory receptor (Brandt et al., J. Exp. Med., 206(7): 1495-1503, 2009). B7H6 was not detected in normal human tissues, but was expressed on human tumor cells.
Accordingly, there is a need in the art for the identification of antibodies capable of inhibiting the interaction of B7H6 with NKp30. There is a therapeutic potential for such antibodies based their ability to modulate co-stimulatory signals and immune responses. Furthermore, antibodies that bind to B7H6 proteins will be useful when conjugated to a cytotoxic agent and used to target a cell expressing B7H6. These and other uses are highly desirable. The present invention provides compositions and methods for these and other uses that should be apparent to those skilled in the art from the teachings herein.